1. Field of the Invention
The present invention relates to a welded portion inspection apparatus and an inspection method thereof, and relates to an inspection apparatus that inspects a welding state of a welded portion formed at the time when a plurality of workpieces is welded by a laser beam, for example, and an inspection method thereof.
2. Description of Related Art
When two steel sheets are put on top of one another and laser beam welding is performed thereon, a quality evaluation is performed on a welded portion formed by the laser beam welding. As an example of such a quality evaluation on the welded portion formed by the laser beam welding, Japanese Patent Application Publication No. 2008-87056 (JP 2008-87056 A) describes a technique to perform a quality evaluation of laser beam welding by use of reflection light of a laser beam.
In a laser beam welding quality determination system described in JP 2008-87056 A, a YAG laser is radiated from a laser torch, for example, and laser reflection light is received by first light receiving output means from a forward-diagonally upward side of a welding proceeding direction. Further, welding light including vapor light (plume) and the laser reflection light is received by second light receiving output means in a direction coaxial to a radiation direction of the laser beam. The laser reflection light and the welding light that are received simultaneously in two predetermined directions are converted into electrical signals according to their respective intensities. This system determines a welding quality based on the signal intensities of the electrical signals or changes thereof.
According to the laser beam welding quality determination system described in JP 2008-87056 A, the laser reflection light and the welding light are received simultaneously in two predetermined directions different from each other and their respective light receiving signal intensities are compared with a threshold set appropriately. Hereby, it is possible to determine occurrence of any one of the following various types of poor welding: weld shrinkage (underfill) in which a weld bead hollows to bury a gap between steel sheets; unjoined weld in which upper and lower steel sheets are not joined due to an excessively large gap between the steel sheets; depressed weld in which a bead is depressed similarly due to an excessively large gap between steel sheets; and molten weld in which a bead disappears accidentally due to fluctuation of a thermal balance; and holed weld.
However, in the laser beam welding quality determination system described in JP 2008-87056 A, in a case where the laser torch is apart from workpieces (steel sheets), for example, the electrical signals obtained from the received laser reflection light and welding light become weak. On that account, determination accuracy of poor welding may decrease. Particularly, in the depressed weld in which a bead is depressed in the laser beam welding, those changes of the electrical signals which are caused due to poor welding decrease. This may cause such a case where poor welding in the workpieces cannot be detected minutely. Further, it is known that vapor light caused due to melting and evaporation of the workpieces and thermal radiation light emitted from a molten pool of the workpieces change according to a workpiece temperature, and the electrical signals obtained from the received laser reflection light and the welding light and the threshold to determine the quality of the laser beam welding change according to the workpiece temperature. Because of this, in a case where the workpiece temperature largely fluctuates in the laser beam welding, the determination accuracy of the poor welding of the workpieces may further decreases.